I've reviewed the four discs comprising this set of Krenek's complete String Quartets, and I refer you to my detailed reviews under the individual entries (Krenek: String Quartet No. 1 & No. 2, String Quartet 3 & 7, String Quartet 4 & 6, String Quartet 5 & 8).
Krenek's eight String Quartets constitute a significant output of works and offer a fascinating testimony of many of the musical styles prevalent in the Pre-2nd World War Austrian-German world, from a burning late Romantic style indebted to Krenek's teacher Franz Schreker, to strict dodecaphony, by way of early Schoenbergian free atonality, Bartokian angularity, Stravinskian neo-classicism and jazz- inspiration. The first seven were written between 1921 (the composer was 21 then) and 1944, the eighth coming as a much later afterthought, in 1980. The language is usually stern, rhythmically vigorous and expressively vehement in the fast movements, and with an intense but stern and angular lyricism in the slow ones. The architecture is often original, usually eschewing the traditional three or four movements. Krenek's part writing is delicate and intricate - no singing first violin against string accompaniment here (except in the 5th Quartet), but each instrument is a "primus inter pares", often in complex canonic or fugal interplay.
The 4th, in its very eclecticism, is probably the most approachable, and the 5th the strangest and maybe the least successful, with its jarring references to Schubert interrupting a first movement of otherwise bartokian vigor, and its surprisingly (for Krenek) gently lyrical but also rambling second and third movements. Overall the two discs with 3 & 7 and 4 & 6 are the best entry points in the series.
I have no other version against which to try and assess the interpretive merits of the Sonare Quartet (there are a small numbers of competing recordings: Ernst Krenek: String Quartets No. 1 Op. 6, No. 7 Op. 96 by the Petersen Quartet on Capriccio, who have also done Quartets 3 & 5 for the same label, available in Germany but not yet listed on this site; Ernst Krenek: String Quartets Nos. 5 & 8 by the Thouvenel Quartet on CRI). However I do have the scores to # 1, 3, 5, 6 and 7, and the situation doesn't seem to me nearly as bad as suggested by the previous reviewer - and by the way only the first half-beat of the beginning of the 6th Quartet has been cut off (which doesn't make it less inexcusable). The Sonare plays with precision, generally fine tonal production, and the recording has afforded superb clarity to the four instruments - an indispensable feature in part writing as intricate and detailed as in Krenek's Quartets. Where I do find the Sonare Quartet wanting though is in their choices of tempos - oftentimes, and especially in the 1st and 6th Quartets, whenever Krenek instructs to play fast or to accelerate, they simply don't. There is nothing that anybody without a score will find chocking, and what they do in those passages is not without some musical sense - but it simply doesn't seem to be what the composer wrote (or blatantly isn't when he has written metronome marks, as in the third movement of the 6th quartet, which under Sonare's bows does not have the quality of a "furious scherzo" described by the composer). In those passages then, the quartets as Krenek wrote them are still to be heard.
But while this set cannot claim to present the ultimate word on these compositions, it does offer, in the absence of anything better, an acceptable and convenient view of Krenek's output in the genre, an interesting if not essential addition to the compositions of Schoenberg, Berg and Hindemith, arguably the three towering masters of Austro-German String Quartet writing in the first half of the 20th Century.